This invention relates to a proportioning apparatus comprising at least one reservoir tank including an outlet port for pourable bulk material and a weighing tank for receiving discharged bulk material, with the receiving tank being arranged under the at least single reservoir tank and including a weighing cell for weighing bulk material, wherein the outlet port of the at least single reservoir tank can be opened or closed by means of a shut-off device.
Proportioning apparatus of this kind are already known and utilized, for example, for proportioning in batch quantities a charging material, composed of either one sort or a plurality of bulk material constituents, to a processing system, for instance, in order to feed extruders with granular plastic material and possibly further admixtures. A distinction is made between volumetric and gravimetric proportioning apparatus, depending on the respective proportioning method to be carried out.
The characteristic of known gravimetric proportioning apparatus is that bulk materials to be proportioned are provided in corresponding reservoir tanks and supplied in batch quantities into a weighing tank arranged under the reservoir tank, according to the proportional weight desired. For this purpose, it is common practice to arrange a shut-off device in the shape of a stop slide valve near the outlet port of each reservoir tank. When the stop slide valve is opened, the bulk material available in the respective reservoir tank will fall into the weighing tank, and then the stop slide valve will be closed again by means of a corresponding control mechanism after having supplied the desired proportional weight into the weighing tank.
Due to the trend of continuously increasing the performance of processing systems for which traditional proportioning apparatus are used and, at the same time, to the progressively increasing requirements for accuracy of production, great demands are made on the precise operation of proportioning apparatus and on the outcome obtained therefrom. Therefore, the shut-off devices arranged between reservoir tanks and the weighing tank play an important key role for accurately weighing the bulk material located in the weighing tank. On the one hand, they need to open and close the outlet ports of the reservoir tanks as quickly as possible and, on the other hand, they need to ensure that the outlet port is entirely sealed, if possible, when in the closed position. Since the shut-off devices are operated at both a high frequency and speed over long periods of time whenever the downstream processing system is activated, high mechanical requirements are established regarding abrasion, stability and the like, higher requirements than those heretofore applied to shut-off devices of proportioning apparatus.
The stop slide valves traditionally used as shut-off devices in the prior art comprise a sliding plate within a rail-like guiding device, which opens or closes the outlet port of the reservoir tank depending on the respective shift position. These conventional stop slide valves having longitudinally shiftable plates turn out to be deficient and susceptible to failures on very high efficiency proportioning apparatus, because they can easily cant within their rail-like guiding device when being actuated very quickly. Furthermore, it cannot always be assured that the outlet port of the reservoir tank is sealed in its entirety. In addition, the structural design of a conventionally constructed proportioning apparatus including reservoir tanks and a weighing tank as well as stop slide valves arranged therebetween, proves to be very complicated; and it may become necessary to retrofit the proportioning apparatus as a result of changes in production requirements or the like, which also turns out to be very difficult.